Re: [PATCH v5 2/6] phy: cdns-dphy: Add Rx support

Hi Vinod,

On Fri, Oct 01, 2021 at 11:53:16AM +0530, Vinod Koul wrote:

On 17-09-21, 22:58, Pratyush Yadav wrote:

quoted

On 16/09/21 12:22PM, Paul Kocialkowski wrote:

quoted

On Fri 03 Sep 21, 00:25, Pratyush Yadav wrote:

quoted

The Cadence DPHY can be used to receive image data over the CSI-2
protocol. Add support for Rx mode. The programming sequence differs from
the Tx mode so it is added as a separate set of hooks to isolate the two
paths. The mode in which the DPHY has to be used is selected based on
the compatible.

I just realized that I didn't follow-up on a previous revision on the debate
about using the phy sub-mode to distinguish between rx/tx.

I see that you've been using a dedicated compatible, but I'm not sure this is a
good fit either. My understanding is that the compatible should describe a group
of register-compatible revisions of a hardware component, not how the hardware
is used specifically. I guess the distinction between rx/tx falls under
the latter rather than the former.

I am not sure if that is the case. For example, we use "ti,am654-ospi" 
for Cadence Quadspi controller. The default compatible, "cdns,qspi-nor", 
only supports Quad SPI (4 lines). The "ti,am654-ospi" compatible also 
supports Octal SPI (8 lines).

Those are hardware defaults right?

quoted

In addition, I feel like the Rx DPHY is almost a different type of 
device from a Tx DPHY. The programming sequence is completely different, 

Is that due to direction or something else..?

quoted

the clocks required are different, etc. So I think using a different 
compatible for Rx mode makes sense.

Is the underlaying IP not capable of both TX and RX and in the specific
situations you are using it as TX and RX.

I am okay that default being TX but you can use Paul's approach of
direction with this to make it better proposal

Given that the RX and TX implementations are very different (it's not a
matter of selecting a mode at runtime), I'm actually tempted to
recommend having two drivers, one for the RX PHY and one for the TX PHY.
This can only be done with two different compatible strings, which I
think would be a better approach.

It's unfortunate that the original compatible string didn't contain
"tx". We could rename it and keep the old one in the driver for backward
compatibility, making things cleaner going forward.

-- 
Regards,

Laurent Pinchart

-- 
linux-phy mailing list
linux-phy@lists.infradead.org
https://lists.infradead.org/mailman/listinfo/linux-phy